It is known that the dermis has high density of capillary blood vessels compared to both the epidermis and the subcutaneous tissue, and has lymph vessel ends. Therefore, directly injected medical agents, in particular, tend to migrate into the blood vessels and/or the lymph vessels, and therefore the absorption speed of the medical agents into the body fluid becomes higher. Particularly, medical agents using macromolecular substance(s) such as hormone, antibody drug, cytokine and the like can be efficiently absorbed into the blood in the dermis. It is also known that the dermis is a place of efficient immunity, which makes it possible to save dose of a vaccine and to strengthen sensitization of a weak vaccine.
Further, it is known that, with respect to a man or woman grown up to some extent, the dermis exists in a certain depth from his or her body surface (i.e., from the surface of his or her stratum corneum). In other words, this fact means that, when injecting a medical agent into the dermis of an adult, it is possible to use a piercing needle having the same length (depth).
Generally, the thickness of the dermis is around 1 mm to mm (average value is 1 mm to 2 mm) in a direction perpendicular to the body surface as a reference. Further, as shown in the cross sectional view of a general skin structure of FIG. 10, the dermis is a layer of skin between an epidermis E and a subcutaneous tissue S, wherein the epidermis E includes a stratum corneum SC and has a thickness of around 0.06 mm to 0.1 mm.
Accordingly, it is difficult to accurately insert a medical agent outlet of the piercing tool (e.g., a needlepoint of the piercing needle) to the dermis between the epidermis and the subcutaneous tissue. If the needlepoint is accidentally inserted to the subcutaneous tissue or the like, the medical agent will fail to be efficiently absorbed.
In recent years, for example, attempts have been made to administer, continuously or in one shot, a medicine using the aforesaid macromolecular substance(s) into the dermis as a target, however the aforesaid problems are prominent in these attempts.
Here, there is known a hypodermic injection device in which the length of the piercing needle to be inserted into the body is defined such that the medical agent is injected into the dermis of the body (see Patent Document 1). Further, there is also known a medical solution injection device in which the depth (insertion depth) of the piercing needle to be inserted into the skin is defined as a predetermined length so that the medical agent is injected into a specific layer of the skin, wherein the piercing needle is inserted into the skin in a direction perpendicular to the body surface (see Patent Document 2).
[Patent Document 1] Japanese Laid-Open Patent Application Publication No. 2001-137343
[Patent Document 1] Japanese Laid-Open Patent Application Publication No. 2005-87519